Not Applicable
Not Applicable
1. Field of the Invention
The present invention relates to hydraulic actuators, and more particularly to hydraulic actuators for operating an intake or exhaust valve for a cylinder of an internal combustion engine.
2. Description of the Related Art
Internal combustion engines have a plurality of cylinders containing pistons that are connected to a crankshaft. Each cylinder has two or more valves to control the air flow into the cylinder and the flow of exhaust gases from the cylinder. Traditionally the cylinder valves were controlled by a cam shaft which in turn was mechanically connected to rotate with the engine crankshaft. Gears, chains, or belts coupled the crankshaft to the cam shaft so that the two would rotate in unison. It is important that the valves open and close at the proper times during the combustion cycle within each cylinder. Heretofore, that timing relationship was fixed by the mechanical coupling between the crankshaft and the cam shaft.
The setting of the cam shaft timing often was a compromise which produced the best overall operation at all engine operating speeds and conditions. However, it was recognized that optimum engine performance could be obtained if the valve timing was varied as a function of engine speed, engine load and other factors.
The trend in motor vehicles is toward the increased use of electronics and microcomputer control systems. This is especially true with respect to controlling the engine, where many mechanical components have been replaced by electrically operated devices controlled by a microcomputer. With this trend, it became possible to determine the optimum engine valve timing based on the operating conditions occurring at any given point and time. That optimum timing then can be used to activate electrically controlled mechanisms which open and close the intake and exhaust valves for each cylinder.
A typical mechanism for this function employs a separate hydraulic actuator to operate the respective intake valve or exhaust valve. A piston, attached to the stem of the cylinder valve, is driven by hydraulic fluid to move the cylinder valve. The existing lubricating oil for the engine frequently is used as the hydraulic fluid and a separate pump supplies that oil at a greater pressure than the conventional oil pump. A solenoid valve, operated by the engine computer, controls the flow of the hydraulic fluid to and from the piston for the cylinder valve. Thus the solenoid actuator does not directly drive the engine valve, but instead operates a valve member to control relatively high pressure fluid that produces movement of the engine valve. This allows a smaller solenoid actuator to be used than where the solenoid alone would have to supply the force that moves the cylinder valve.
A hydraulic actuator for operating an engine cylinder valve includes a driver piston to move the engine cylinder valve into open and closed states. A hydraulic valve is in fluid communication with the driver piston, a first conduit carrying fluid at a first pressure, and a second conduit carrying fluid at a second pressure that is less than the first pressure. For example, the second conduit may be connected to a fluid reservoir for the engine. The hydraulic valve has a valve spool which in a first position enables fluid to flow between the first conduit and the driver piston to open the engine cylinder valve, and in a second position enables fluid to flow between the second conduit and the driver piston to close the engine cylinder valve.
An operator, such as an electrically driven solenoid, is operably coupled to produce movement of the valve spool into the first and second positions. A feedback mechanism is coupled to the valve spool, The feedback mechanism responds to movement of the driver piston by moving the valve spool into a third position at which neither the first conduit nor the second conduit is in fluid communication with the driver piston. The feedback mechanism ensures that the stroke of the hydraulic actuator is proportional to the magnitude of the electric current applied to the operator regardless of variation of the pressure in the first conduit.
In one embodiment of the hydraulic actuator, the feedback mechanism comprises a feedback piston which moves in response to fluid pressure produced by movement of the drive piston. A feedback spring extends between the valve spool and the feedback piston. In another embodiment, the drive piston slides within a common bore with the valve spool and the feedback mechanism comprises a feedback spring which extends between the valve spool and the drive piston.